Following DNA damage or inhibition of DNA synthesis, Escherichia coli exhibits a variety of new processes, such as prophage induction, new repair capacity, and mutagenesis. The products of two genes, recA and lexA, are required for expression of these processes. We have identified the lexA gene product and are currently purifying it. We intend to test the model that the lexA gene product is a repressor of the recA gene, as well as other models which involve the lexA gene in the regulation of the recA gene.